Discussion
Pollen morphology of species of Sorbus s.lat., Aria and
their hybrids in Norway (Hedlundia ), is rarely studied in detail.
The first European studies of pollen morphology of Sorbus in
general, was Erdtman (1952) and Praglowski (1963), later followed by
Eide (1981). These studies were novel as they believed thatSorbus pollen could be identified, and not only categorized asCrataegus -type of pollen as earlier. Later, Boyd and Dickson
(1987) studied pollen of S. aucuparia (2n), A. rupicola(4n), H. arranensis (3n), and H. pseudofennica (4n), using
LM. They found that S. aucuparia could be separated from the
other taxa but found less differences between the hybrid species.
Bednorz et al. (2003) found that while size measures were overlapping
for species and hybrids of Sorbus , exine sculpturing provided
features for taxonomic classifications. There has been a general opinion
amongst palynologists that pollen from the family Rosaceae only is
useful for morphological identification on higher ranks, like subfamily
or genus. Wronska-Pilarek et al. (2022) points out that studies report
useful morphological characters for species delimitation in the
Rosaceae, such as the length of the polar axis (P), pollen shape (P/E),
operculum structure, and presence/lack of costae colpi.
Bednorz et al. (2005) concludes that “… it is relatively easy to
separate pollen grains” of the genera Sorbus ,Chamaemespilus , and Torminalis using SEM. They found that
type of sculpture, length of polar axis (P), number of ectocolpi, and
ratio of the ectexine to endexine thickness are the most important
separating characters for these genera. These taxa were previously
defined within the genus Sorbus but are now classified as genera
(Sennikov and Kurtto, 2017).
Our results show that pollen of Aria generally has a longer polar
axis (P) than pollen of Sorbus and Hedlundia (Fig. 3).
From the PCA analysis (Fig. 7), Aria has clearly larger pollen
size in general, compared to Sorbus and Hedlundia . AsAria and Sorbus are not closely related (Lo and Donoghue,
2012), but Aria is related to Malus , whereas Sorbusare related to Pyrus , it is expected to see differences between
these genera, and the main separating character is size, and more
specific, the length of the longest axis (P). For the hybrids
(Hedlundia ), it has been hypothesized that Sorbus is the
ovule donor (Liljefors, 1955), and therefore it’s reasonable to believe
that pollen mostly resembles their mother in size.
Pollen size was categorized in different intervals by Erdtman (1969).
Pollen with polar axis (P) between 50-100 µm are large, 25-50 µm are
medium sized, and between 10-25 µm are defined as small. From our
measurements, A. edulis , H. hybrida 1 and A.
obtusifolia have a mean P value above 25 µm and are therefore medium
sized. All other specimens in our study have small pollen. With its
medium sized pollen, H. hybrida 1, stands out as very large
compared to all other species of Hedlundia . H. hybrida is
a morphologically very variable species, and different subspecies have
been suggested (Grundt and Salvesen, 2011). This variation is also seen
here, when comparing the two different specimens of H. hybrida in
our study. In the other end of the range are H. neglecta , with
very tiny pollen.
We have compared our measurements of the shape of pollen (ratio P/E)
with other available studies and see that there is a large range between
studies (see Figure 8). In general, our samples have a higher ratio than
in all the other studies, whereas Eide (1981) has lower ratios than the
others. This can be related to different preparation techniques.
According to Katsiotis and Forsberg (1995) pollen size can be influenced
by treatment methods and mounting media. Pollen grains tend to swell in
glycerol (Moore and Webb, 1978). This can lead to different sizes in
different studies and can explain why our measured pollen has a higher
ratio as the samples are all mounted in glycerol.
S. aucuparia pollen ranges from a ratio of 0.96 in Eide (1981) to
1.48 in this study (Figure 8). This gives rise to all forms of pollen;
oblate spheroidal form (Eide, 1981), prolate-spherioidal shape (Bednorz
et al., 2003; Boyd and Dickson, 1987; Erdtman et al., 1961), subprolate
shape (Bednorz et al., 2005; Praglowski, 1963), and prolate shape in
this study. S. aucuparia is a morphological extreme variable
species, and many have suggested establishing subspecies to account for
this variation. This may also be reflected through this study of pollen
shape.
For H. hybrida , the variation in P/E is lower, ranging from 1.05
to 1.36, but still giving rise to three different shape forms. H.
hybrida is also a very variable species, resulting from hybridization
events with different parent trees. Our two samples of H. hybridaoriginates from different parts of Norway, H. hybrida 1 from
Telemark (4), eastern Norway, whereas H. hybrida 2 is collected
in Sogndal (5) in western Norway and is a morphological very different
type. From the comparison of different data on the shape ofSorbus pollen, we can conclude that shape is only useful where
the methodology is consistent.
Erdtman (1952) hypothesized that pollen size is dependent on polyploidy.
Erdtman (1969) writes that “Pollen grains and spores in polyploid
species are, as a rule, larger than in diploids”. In general, it is
argued that larger amount of DNA per cell will make larger cells, both
for pollen and spores. Several examples of species with larger pollen
size correlated with ploidy levels are shown, as for Andropogonby Gould (1957), for Avena species studied by Katsiotis and
Forsberg (1995), and Altmann et al. (1994) even used pollen size to
determine the ploidy level of Arabidopsis thaliana .
After the “Eukaryotic genome size databases” (Gregory et al., 2007)
was launched, Knight et al. (2010) used this database together with
pollen size data from different sources to study the relation between
pollen and genome size. They found that from 464 species, there was a
positive correlation between pollen width (E) and genome size. Based on
literature, Knight et al. (2010) estimated that pollen size increase by
1.1x-2x with double C-value (amount of DNA in a haploid genome).
When plotting our mean pollen width (E) against ploidy level (DNA
amount) we find a positive correlation of 0.45, as shown in Figure 2
(small box). This supports the hypothesis that more DNA results in
larger pollen, hence pollen of polyploids are thicker than diploids.
Eide (1981) argued that exine ornamentation was one of the most
important features not only to identify the pollen of Sorbus , but
also the Rosaceae. Bednorz et al. (2003) finds that the exine
sculpturing was the main character for distinguishing pollen fromS. aucuparia , H. arranensis , H. hybrida , andH. mougeotii , and concluded that S. arranensis was
distinct from the other species by its short rugulate-like exine
pattern. Both S. hybrida and S. mougeotii were
characterized by very long striation, with the striae clearly thinner atS. mougeotii . The S. aucuparia pollen had shorter striae
and dense, well visible perforations between vallae.
The variation in ornamentation of our samples is shown in the SEM
pictures given in Figure 5. The variation in surface structures is
clearly visible and a basis for classification of the taxa included in
this study. Perforations is variable, and a basis of our key to identify
Norwegian species:
Medium sized pollen (25-50 µm)
Few perforations Aria obtusifolia
Medium perforations Aria edulis
Dense perforations Aria rupicola
Small pollen (10-25 µm)
Few perforations
Polar view circular
Medium length of striae Hedlundia sognensisShort striae Hedlundia subpinnata Polar view triangular Hedlundia neglecta
Medium perforations
Short striae Hedlundia hybridaLong striae Hedlundia mougeotii
Dense perforations
Circular polar view
Short striae in parallel directions Hedlundia subarranensisMedium striae in variable directions Hedlundia subsimilisLong striae in parallel directions Sorbus aucuparia Triangular polar view
Striate-rugulate exine Hedlundia lancifoliaStriate exine Hedlundia meinichii
It is possible to identify pollen of Sorbus , Aria andHedlundia with morphological characters using LM only, but using
SEM will ease the identification. The identification will need a very
close study of the given characters but will be feasible.
This study of pollen morphology of Sorbus and Aria with
their hybrids (Hedlundia ), shows that the genus Aria has
longer pollen than both Sorbus and Hedlundia , and that
these are separated by size. Sorbus pollen is smaller thanHedlundia but with some overlap. By using size in combination
with several characters of surface ornamentation, hybrids are possible
to identify by pollen morphology.